1. Technical Field
The invention relates to organic thin film transistors (organic TFT).
2. Discussion of Related Technologies
The development of polyacetylene, a conjugated organic polymer having semiconductor characteristics, has opened the possibility to use organic polymer transistors in electronic and optical devices. The organic polymer is advantageous in that it can be synthesized using various methods, can be readily molded into a film shape, has good flexibility and electric conductivity, and has a low production cost.
A conventional silicon TFT includes a semiconductor layer formed of silicon, source and drain regions doped with a high concentration dopant, a channel region between the source region and the drain region, a gate electrode disposed on a region corresponding to the channel region and insulated from the semiconductor layer, and source and drain electrodes respectively connected to the source and drain regions.
Some disadvantages of conventional silicon TFTs are that they are expensive to manufacture and can easily break by an external impact. Furthermore, the manufacturing process requires high temperatures (over 300° C.), which precludes the use of a plastic substrate.
In flat displays such as liquid crystal displays (LCDs), or electroluminescence displays (ELD), TFTs are used as a switching device that controls the operation of each pixel and a driving device for driving each pixel. It has been attempted to use a plastic (opposed to glass) substrate to produce flat panel displays that are large, thin, and possess the desired flexibility characteristics. However, when the plastic substrate is used, a lower temperature process must be employed, which precludes the use of conventional silicon TFTs.
The use of an organic material for the semiconductor layer may help resolve this because it can be formed at lower temperatures.
However, the characteristics of a plurality of simultaneously manufactured TFTs that include an organic semiconductor layer may not be identical with respect to each of the TFTs.
FIG. 1 is a cross-sectional view illustrating an inverted coplanar type organic TFT. FIGS. 2A through 2C are conceptual cross-sectional views that illustrate a conventional coating process for forming an organic semiconductor layer using a spin coating method. FIG. 3A is a plan view that illustrates, in conjunction with FIGS. 3B and 3C, how physical characteristics within the organic semiconductor layer formed by this method may vary at different locations within the organic semiconductor layer. FIGS. 3B and 3C are enlarged plan views of respective portion A and portion B of the organic semiconductor layer of FIG. 3A.
Referring to FIG. 1, an organic semiconductor layer 30 covering a gate electrode 20, a gate insulating film 60, a source electrode 40, and a drain electrode 50 are formed over a substrate 10 using a spin coating method as depicted in FIGS. 2A through 2C. Referring to FIGS. 2A through 2C, an organic semiconductor layer 3 is formed by dropping an organic material 3a on the substrate 1 through a nozzle 2. The substrate 1 is rotated to create a centrifugal force that distributes the material 3a across the surface of the substrate 1, thereby forming the organic semiconductor layer.
However, the spin coating method creates an organic semiconductor layer 3 that is non-uniform. As depicted in FIG. 3A, spin coating may cause the organic semiconductor layer 3 to form in multiple directions. For example, portions of the organic semiconductor layer 3 at different locations, such as those represented by A and B in FIGS. 3A-3C, may be formed with the material in each portion having an orientation in a different direction than any other given portion.
Portions of organic TFTs formed in different directions, such as those represented by A and B, have different characteristics of threshold voltage. As a result, when a flat panel display is manufactured using organic TFTs formed by spin coating, high-resolution images according to the input image signals cannot be displayed.